Who am I:
Plat5 at the moment. I used to be a one trick guy in S4 and carried myself to Diamond with Nasus.

Who is Nasus?
****. That's about it. You're weak in teamfights (you'll get kited), you don't bring any CC and your early game sucks. You're not a late game beast because you can't reach anyone - your mid-game is really strong though. Splitpushing and small skirmishes is what you excel at.

You have so little mobility and skillshots that there's no way to outplay any other champ. What you need to do is dodge as much as possible of their skills and only when they make a mistake - you can capitalize on that.
Vs squishy champions (Riven, Fiora etc.) you want to trade back even in early levels. AA+Q is probably the most you'll ever get.
Vs champs like Renekton, Gragas (lots of sustain) you're better of just taking their combo and continuing farming.
You max E if you're ****ed or vs ranged (Teemo, Kennen etc.).
You max Q for CDR and when you can actually use it.
You always start Q into W and only then E. Of course max R.

Random solid piece of advice - if you're not sure if you're gonna die or survive - use R, don't get tempted. It's not that strong offensively anyway.

It's going to be awkward a lot of times in even match-ups. Your 1st priority is not to die so therefore you'll end up rushing items like Glacial Shroud, Spectre's Crowl or Kindlegem. At the same time you want to rush Sheen and Trinity force. So usually what happens is this:
You're slightly/heavily losing - you rush full tanky item 1st then continue to Trinity Force.
It's completely even - you get some tanky stats (one of these: Glacial Shroud, Spectre's Crowl or Kindlegem) and get Sheen. Reevaluate afterwise and go for Trinity Force or tanky item depending.
You're winning - you get some tanky stats (one of these: Glacial Shroud, Spectre's Crowl or Kindlegem) and then Sheen into Trinity Force.
Here's the exception - if you can already see that you won't be able to splitpush late game (i.e. their team has heavy engage and your team lacks disengage) then you should be building full tank and going for Iceborn Gauntlet instead of Trinity force. It does make your build path waaaaay better but your 1v1 gets weaker.

Your strongest part of the game. You should be able to 1v1 just about anyone (**** Yasuo and Darius though) or at least nobody can deny you farm. You're crazy strong in skirmishes like 2v2 or 3v3.

Preferably you want to continue splitting. Nasus doesn't bring much in teamfights but you can take down turrets fast. In teamfights you're just a frontline who soaks up the damage. If you don't have any hard engage (Amumu, Vi), you should've built full tank with Gargoyle Stoneplate and peeling for your carries.